Templar is the best spec in the game, by far.
1. Spell Purge is an AoE eldritch detonator, and is absurdly powerful because it's considered a Mage ability. Eldritch detonators do a flat 500% weapon damage when detonating a Warrior or Rogue ability, and Warrior weapon damage is triple that of Mages, so you get 3x the damage using it to detonate a status effect versus using Dispel. You can also prime your own detonator with Wrath of Heaven and get the full cross-class damage bonus because Spell Purge is coded exactly like Dispel. You can utterly wipe out entire groups with this. If you have an Assassin to open with Sleep you can Spell Purge the asleep targets for thousands of damage + fear.
2. Applying Weakness to a Shocked target puts it to sleep. Have your Mage open with Chain Lightning and Maker's Will now gives every basic attack and every ability used from every party member a 5% chance to sleep the target. Sleep lasts for 3 seconds after being broken by damage and can be detonated during these 3 seconds. Whenever you get a proc, spell purge or dispel will do tons of damage and fear the target, shield bash will apply a huge rupture DoT.
The Templar synergies absolutely destroys shit in this game. I'm running a Mage char and I use Solas as well. Solas opens with Chain Lightning -> Veilstrike which puts all the enemies to sleep. Cassandra spell purges for thousands of damage and fears the entire pack, I have my main character lay down Static Cage which causes all the feared enemies to get paralyzed in the center as they try to run out, hit the closely packed, paralyzed group with Shield Bash to set off an AoE discharge for every enemy hit for thousands of damage again. If anything is still alive use Wrath of Heaven to AoE stun and then Dispel the group for another massive damage combo and fearing everything all over again. Everything will get paralyzed in the center again and you can use Shadow Strike or upgraded Long Shot to detonate every enemy in the group again. When they get up from that, War Horn to fear everything back out again.